As image sensors are incorporated into low cost cameras and other devices, such as mobile cellular telephones and personal digital assistants having a main function that may not be capturing images, it is preferable to minimize the cost of the image sensor. Image sensors usually require a focussing lens as part of an imaging assembly so that a representation of a subject may be reproduced. In addition, the imaging assembly may incorporate other components such as additional lens elements, an aperture and an infra-red (IR) filter. Typically, prior art imaging assemblies mount individual components in a support structure commonly referred to as a barrel. The lenses and barrel are usually made from low cost materials, such as plastic. Generally, prior-art imaging assemblies of this type are intentionally low cost and can suffer from problems of limited chromatic correction and difficulties surviving elevated temperatures.
Typically, chromatic correction is required because a lens will not focus light of different wavelengths on the same point. Focal length depends on refraction, and the index of refraction for short wavelengths (for example, blue light) is larger than that of longer wavelengths (for example, red light). This is termed chromatic aberration of a lens.
It may also be beneficial for an imaging assembly to be able to withstand elevated temperatures. One reason for this is that solid state image sensors are often subjected to a re-flow solder process. The re-flow solder process requires that the image sensor be passed through a re-flow oven that melts solder deposits to electrically connect various elements. In a fixing operation, prior to passing the image sensor through a reflow oven, components of the image sensor are fixed in place. Because the imaging assembly cannot tolerate the temperature of the reflow oven, it is fixed in place in a separate fixing operation. It would be advantageous if only one fixing operation was needed for all components.
Typically, the components associated with prior art barrel imaging assemblies have optical properties that are modified by elevated temperature, and, therefore may not be suitable for a re-flow solder process.